Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications, such as angina pectoris and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory Parkinson's Disease, and DBS has also recently been applied in additional areas, such as essential tremor and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Each of these implantable neurostimulation systems typically includes one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. Thus, electrical pulses can be delivered from the neurostimulator to the stimulation electrode(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. A typical stimulation parameter set may include the electrodes that are sourcing (anodes) or returning (cathodes) the stimulation current at any given time, as well as the amplitude, duration, and rate of the stimulation pulses. The neurostimulation system may further comprise a handheld patient programmer to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The handheld programmer in the form of a remote control (RC) may, itself, be programmed by a clinician, for example, by using a clinician's programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
When stimulating neural tissue, the order in which nerve fibers are electrically stimulated or activated (i.e., the neural recruitment order), which is governed by spatial and morphometric criteria, has been a known issue that can limit efficacy by resulting in side effects (e.g., dorsal root stimulation, motor fiber stimulation, non-root-related effects, such as temperature, proprioceptor, reflex arc nerves, etc) that preclude the programming of stimulation systems to recruit fibers that could have possibly increased efficacy of the therapy.
For example, the neural recruitment order may be correlated to the diameter of the nerve fibers that innervate the volume of tissue to be stimulated. In SCS, activation (i.e., recruitment) of large diameter sensory fibers is believed to reduce/block transmission of smaller diameter pain fibers via interneuronal interaction in the dorsal horn of the spinal cord. Activation of large sensory fibers also creates a sensation known as paresthesia that can be characterized as an alternative sensation that replaces the pain signals sensed by the patient.
Because larger nerve fibers have lower stimulation thresholds than smaller nerve fibers, the larger nerve fibers will normally be stimulated before smaller nerve fibers when located the same distance from the active electrode or electrodes. Because of this, over-stimulation of nerve fibers closest to the active electrode(s) is often unavoidable, thereby leading to uncomfortable, intense sensations in unwanted areas, and in the case of SCS, preventing the recruitment of deeper and/or smaller nerve fibers that might increase the efficacy of the therapy.
Thus, a neurostimulation system that could modify the recruitment order with respect to depth of nerve fibers, such that deeper nerve fibers are recruited more preferentially than shallower nerve fibers, would be valuable to “tune” the desired therapeutic effect of a neurostimulation application, such as SCS.